Long-term burning in penis. Tests all normal.

Hi everyone,
For most of my adolescent life (im 20 now), I can remember having a burning sensation in the tip of my penis, at the urethral opening. It generally happens following urination, and the area has an unpleasant odor, as well as remaining wet (it's as if some urine still remains within the urethra even after I finish, that leaks out afterwards) As far as im aware, ive never noticed any discharge or other signs of a UTI apart from the burning. I've had a urine test done around a year ago after telling my doctor about the problem, with no signs of any problems being detected. This problem predates my sex life, so cant be an STI, though I have had unprotected sex through the problem (with one long term partner). It has not got better or worse, just remained. My water intake has not always been great - perhaps 1 and a half liters at best for a number of years, but i am trying to increase it. Would I be right in assuming this is a cause?
Cheers for any help.